It sits in Additional Content & Scripts box in Checkout configuration. It's some kind of analytics service but I don't think that it's either sumAll or MarketBrain that I'm using. Definitely should've commented this.

Does anyone know what runs on sl-promoserver.herokuapp.com? The name makes me wonder if this was to be just a temporary mechanism for collecting data. I wonder if it does indeed belong to sumAll or MarketBrain.